[PATCH v13 09/13] x86/um: nommu: signal handling

Hajime Tazaki
2025-11-08T08:06:18Z

This commit updates the behavior of signal handling under !MMU
environment. It adds the alignment code for signal frame as the frame
is used in userspace as-is.

floating point register is carefully handling upon entry/leave of
syscall routine so that signal handlers can read/write the contents of
the register.

It also adds the follow up routine for SIGSEGV as a signal delivery runs
in the same stack frame while we have to avoid endless SIGSEGV.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
---
arch/um/include/shared/kern_util.h | 4 +
arch/um/nommu/Makefile | 2 +-
arch/um/nommu/os-Linux/signal.c | 8 +
arch/um/nommu/trap.c | 201 ++++++++++++++++++++++++++
arch/um/os-Linux/signal.c | 3 +-
arch/x86/um/nommu/do_syscall_64.c | 6 +
arch/x86/um/nommu/os-Linux/mcontext.c | 11 ++
arch/x86/um/shared/sysdep/mcontext.h | 1 +
arch/x86/um/shared/sysdep/ptrace.h | 2 +-
9 files changed, 235 insertions(+), 3 deletions(-)
create mode 100644 arch/um/nommu/trap.c

diff --git a/arch/um/include/shared/kern_util.h b/arch/um/include/shared/kern_util.h
index 7798f16a4677..46c8d6336ca1 100644
--- a/arch/um/include/shared/kern_util.h
+++ b/arch/um/include/shared/kern_util.h
@@ -70,4 +70,8 @@ void um_idle_sleep(void);

void kasan_map_memory(void *start, size_t len);

+#ifndef CONFIG_MMU
+extern void nommu_relay_signal(void *ptr);
+#endif
+
#endif
diff --git a/arch/um/nommu/Makefile b/arch/um/nommu/Makefile
index baab7c2f57c2..096221590cfd 100644
--- a/arch/um/nommu/Makefile
+++ b/arch/um/nommu/Makefile
@@ -1,3 +1,3 @@
# SPDX-License-Identifier: GPL-2.0

-obj-y := os-Linux/
+obj-y := trap.o os-Linux/
diff --git a/arch/um/nommu/os-Linux/signal.c b/arch/um/nommu/os-Linux/signal.c
index 19043b9652e2..6febb178dcda 100644
--- a/arch/um/nommu/os-Linux/signal.c
+++ b/arch/um/nommu/os-Linux/signal.c
@@ -5,6 +5,7 @@
#include <os.h>
#include <sysdep/mcontext.h>
#include <sys/ucontext.h>
+#include <as-layout.h>

void sigsys_handler(int sig, struct siginfo *si,
struct uml_pt_regs *regs, void *ptr)
@@ -14,3 +15,10 @@ void sigsys_handler(int sig, struct siginfo *si,
/* hook syscall via SIGSYS */
set_mc_sigsys_hook(mc);
}
+
+void nommu_relay_signal(void *ptr)
+{
+ mcontext_t *mc = (mcontext_t *) ptr;
+
+ set_mc_relay_signal(mc);
+}
diff --git a/arch/um/nommu/trap.c b/arch/um/nommu/trap.c
new file mode 100644
index 000000000000..430297517455
--- /dev/null
+++ b/arch/um/nommu/trap.c
@@ -0,0 +1,201 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/mm.h>
+#include <linux/sched/signal.h>
+#include <linux/hardirq.h>
+#include <linux/module.h>
+#include <linux/uaccess.h>
+#include <linux/sched/debug.h>
+#include <asm/current.h>
+#include <asm/tlbflush.h>
+#include <arch.h>
+#include <as-layout.h>
+#include <kern_util.h>
+#include <os.h>
+#include <skas.h>
+
+/*
+ * Note this is constrained to return 0, -EFAULT, -EACCES, -ENOMEM by
+ * segv().
+ */
+int handle_page_fault(unsigned long address, unsigned long ip,
+ int is_write, int is_user, int *code_out)
+{
+ /* !MMU has no pagefault */
+ return -EFAULT;
+}
+
+static void show_segv_info(struct uml_pt_regs *regs)
+{
+ struct task_struct *tsk = current;
+ struct faultinfo *fi = UPT_FAULTINFO(regs);
+
+ if (!unhandled_signal(tsk, SIGSEGV))
+ return;
+
+ pr_warn_ratelimited("%s%s[%d]: segfault at %lx ip %p sp %p error %x",
+ task_pid_nr(tsk) > 1 ? KERN_INFO : KERN_EMERG,
+ tsk->comm, task_pid_nr(tsk), FAULT_ADDRESS(*fi),
+ (void *)UPT_IP(regs), (void *)UPT_SP(regs),
+ fi->error_code);
+}
+
+static void bad_segv(struct faultinfo fi, unsigned long ip)
+{
+ current->thread.arch.faultinfo = fi;
+ force_sig_fault(SIGSEGV, SEGV_ACCERR, (void __user *) FAULT_ADDRESS(fi));
+}
+
+void fatal_sigsegv(void)
+{
+ force_fatal_sig(SIGSEGV);
+ do_signal(&current->thread.regs);
+ /*
+ * This is to tell gcc that we're not returning - do_signal
+ * can, in general, return, but in this case, it's not, since
+ * we just got a fatal SIGSEGV queued.
+ */
+ os_dump_core();
+}
+
+/**
+ * segv_handler() - the SIGSEGV handler
+ * @sig: the signal number
+ * @unused_si: the signal info struct; unused in this handler
+ * @regs: the ptrace register information
+ *
+ * The handler first extracts the faultinfo from the UML ptrace regs struct.
+ * If the userfault did not happen in an UML userspace process, bad_segv is called.
+ * Otherwise the signal did happen in a cloned userspace process, handle it.
+ */
+void segv_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs,
+ void *mc)
+{
+ struct faultinfo *fi = UPT_FAULTINFO(regs);
+
+ /* !MMU specific part; detection of userspace */
+ /* mark is_user=1 when the IP is from userspace code. */
+ if (UPT_IP(regs) > uml_reserved && UPT_IP(regs) < high_physmem)
+ regs->is_user = 1;
+
+ if (UPT_IS_USER(regs) && !SEGV_IS_FIXABLE(fi)) {
+ show_segv_info(regs);
+ bad_segv(*fi, UPT_IP(regs));
+ return;
+ }
+ segv(*fi, UPT_IP(regs), UPT_IS_USER(regs), regs, mc);
+
+ /* !MMU specific part; detection of userspace */
+ relay_signal(sig, unused_si, regs, mc);
+}
+
+/*
+ * We give a *copy* of the faultinfo in the regs to segv.
+ * This must be done, since nesting SEGVs could overwrite
+ * the info in the regs. A pointer to the info then would
+ * give us bad data!
+ */
+unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
+ struct uml_pt_regs *regs, void *mc)
+{
+ int si_code;
+ int err;
+ int is_write = FAULT_WRITE(fi);
+ unsigned long address = FAULT_ADDRESS(fi);
+
+ if (!is_user && regs)
+ current->thread.segv_regs = container_of(regs, struct pt_regs, regs);
+
+ if (current->mm == NULL) {
+ show_regs(container_of(regs, struct pt_regs, regs));
+ panic("Segfault with no mm");
+ } else if (!is_user && address > PAGE_SIZE && address < TASK_SIZE) {
+ show_regs(container_of(regs, struct pt_regs, regs));
+ panic("Kernel tried to access user memory at addr 0x%lx, ip 0x%lx",
+ address, ip);
+ }
+
+ if (SEGV_IS_FIXABLE(&fi))
+ err = handle_page_fault(address, ip, is_write, is_user,
+ &si_code);
+ else {
+ err = -EFAULT;
+ /*
+ * A thread accessed NULL, we get a fault, but CR2 is invalid.
+ * This code is used in __do_copy_from_user() of TT mode.
+ * XXX tt mode is gone, so maybe this isn't needed any more
+ */
+ address = 0;
+ }
+
+ if (!err)
+ goto out;
+ else if (!is_user && arch_fixup(ip, regs))
+ goto out;
+
+ if (!is_user) {
+ show_regs(container_of(regs, struct pt_regs, regs));
+ panic("Kernel mode fault at addr 0x%lx, ip 0x%lx",
+ address, ip);
+ }
+
+ show_segv_info(regs);
+
+ if (err == -EACCES) {
+ current->thread.arch.faultinfo = fi;
+ force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)address);
+ } else {
+ WARN_ON_ONCE(err != -EFAULT);
+ current->thread.arch.faultinfo = fi;
+ force_sig_fault(SIGSEGV, si_code, (void __user *) address);
+ }
+
+out:
+ if (regs)
+ current->thread.segv_regs = NULL;
+
+ return 0;
+}
+
+void relay_signal(int sig, struct siginfo *si, struct uml_pt_regs *regs,
+ void *mc)
+{
+ int code, err;
+
+ /* !MMU specific part; detection of userspace */
+ /* mark is_user=1 when the IP is from userspace code. */
+ if (UPT_IP(regs) > uml_reserved && UPT_IP(regs) < high_physmem)
+ regs->is_user = 1;
+
+ if (!UPT_IS_USER(regs)) {
+ if (sig == SIGBUS)
+ pr_err("Bus error - the host /dev/shm or /tmp mount likely just ran out of space\n");
+ panic("Kernel mode signal %d", sig);
+ }
+ /* if is_user==1, set return to userspace sig handler to relay signal */
+ nommu_relay_signal(mc);
+
+ arch_examine_signal(sig, regs);
+
+ /* Is the signal layout for the signal known?
+ * Signal data must be scrubbed to prevent information leaks.
+ */
+ code = si->si_code;
+ err = si->si_errno;
+ if ((err == 0) && (siginfo_layout(sig, code) == SIL_FAULT)) {
+ struct faultinfo *fi = UPT_FAULTINFO(regs);
+
+ current->thread.arch.faultinfo = *fi;
+ force_sig_fault(sig, code, (void __user *)FAULT_ADDRESS(*fi));
+ } else {
+ pr_err("Attempted to relay unknown signal %d (si_code = %d) with errno %d\n",
+ sig, code, err);
+ force_sig(sig);
+ }
+}
+
+void winch(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs,
+ void *mc)
+{
+ do_IRQ(WINCH_IRQ, regs);
+}
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 2f6795cd884c..28754f56c42b 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -41,9 +41,10 @@ static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
int save_errno = errno;

r.is_user = 0;
+ if (mc)
+ get_regs_from_mc(&r, mc);
if (sig == SIGSEGV) {
/* For segfaults, we want the data from the sigcontext. */
- get_regs_from_mc(&r, mc);
GET_FAULTINFO_FROM_MC(r.faultinfo, mc);
}

diff --git a/arch/x86/um/nommu/do_syscall_64.c b/arch/x86/um/nommu/do_syscall_64.c
index 9bc630995df9..cf5a347ee9b1 100644
--- a/arch/x86/um/nommu/do_syscall_64.c
+++ b/arch/x86/um/nommu/do_syscall_64.c
@@ -44,6 +44,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
/* set fs register to the original host one */
os_x86_arch_prctl(0, ARCH_SET_FS, (void *)host_fs);

+ /* save fp registers */
+ asm volatile("fxsaveq %0" : "=m"(*(struct _xstate *)regs->regs.fp));
+
if (likely(syscall < NR_syscalls)) {
unsigned long ret;

@@ -61,6 +64,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
/* handle tasks and signals at the end */
interrupt_end();

+ /* restore fp registers */
+ asm volatile("fxrstorq %0" : : "m"((current->thread.regs.regs.fp)));
+
/* restore back fs register to userspace configured one */
os_x86_arch_prctl(0, ARCH_SET_FS,
(void *)(current->thread.regs.regs.gp[FS_BASE
diff --git a/arch/x86/um/nommu/os-Linux/mcontext.c b/arch/x86/um/nommu/os-Linux/mcontext.c
index b62a6195096f..afa20f1e235a 100644
--- a/arch/x86/um/nommu/os-Linux/mcontext.c
+++ b/arch/x86/um/nommu/os-Linux/mcontext.c
@@ -4,10 +4,21 @@
#include <asm/ptrace.h>
#include <sysdep/ptrace.h>
#include <sysdep/mcontext.h>
+#include <os.h>
+#include "../syscalls.h"

extern long __kernel_vsyscall(int64_t a0, int64_t a1, int64_t a2, int64_t a3,
int64_t a4, int64_t a5, int64_t a6);

+void set_mc_relay_signal(mcontext_t *mc)
+{
+ /* configure stack and userspace returning routine as
+ * instruction pointer
+ */
+ mc->gregs[REG_RSP] = (unsigned long) current_top_of_stack;
+ mc->gregs[REG_RIP] = (unsigned long) userspace;
+}
+
void set_mc_sigsys_hook(mcontext_t *mc)
{
mc->gregs[REG_RCX] = mc->gregs[REG_RIP];
diff --git a/arch/x86/um/shared/sysdep/mcontext.h b/arch/x86/um/shared/sysdep/mcontext.h
index 9a0d6087f357..82a5f38b350f 100644
--- a/arch/x86/um/shared/sysdep/mcontext.h
+++ b/arch/x86/um/shared/sysdep/mcontext.h
@@ -19,6 +19,7 @@ extern int set_stub_state(struct uml_pt_regs *regs, struct stub_data *data,

#ifndef CONFIG_MMU
extern void set_mc_sigsys_hook(mcontext_t *mc);
+extern void set_mc_relay_signal(mcontext_t *mc);
#endif

#ifdef __i386__
diff --git a/arch/x86/um/shared/sysdep/ptrace.h b/arch/x86/um/shared/sysdep/ptrace.h
index 572ea2d79131..6ed6bb1ca50e 100644
--- a/arch/x86/um/shared/sysdep/ptrace.h
+++ b/arch/x86/um/shared/sysdep/ptrace.h
@@ -53,7 +53,7 @@ struct uml_pt_regs {
int is_user;

/* Dynamically sized FP registers (holds an XSTATE) */
- unsigned long fp[];
+ unsigned long fp[] __attribute__((aligned(16)));
};

#define EMPTY_UML_PT_REGS { }
--
2.43.0